The field of the invention is dual frequency coil pairs for receiving nuclear magnetic resonance signals, where each individual coil is tuned to a different resonant frequency.
Given that the magnetic field conditions for nuclear magnetic resonance, which are well known in the art, are satisfied, the nuclear magnetic resonance phenomenon occurs at a unique characteristic frequency, referred to in the art as the Larmor frequency, which is dependent upon the specific nucleus of interest. The specific nucleus to be examined is dependant upon the particular application. For example, the two most common types of nuclear magnetic resonance examinations are imaging and spectroscopy. NMR imaging is used to acquire a composite spatial image by repetitively localizing the NMR phenomenon to small picture elements (pixels) within an area of interest. Another separate application of the nuclear magnetic resonance phenomenon is that of NMR spectroscopy. The field of NMR spectroscopy is well known in the art, and deals with performing a detailed analysis of the NMR signal in the frequency domain, again for a particular area of interest.
A problem arises in performing NMR spectroscopy in that it is necessary to first localize the NMR phenomenon to the area of interest in which the spectroscopy is to be performed. In practice, this localization is performed by first using the NMR apparatus in an imaging mode to acquire an image for verifying the spatial coordinates of the area which is to be used for the subsequent spectroscopy. Having established the correct spatial coordinates through NMR imaging, the NMR apparatus is changed to operate in a spectroscopy mode, and the desired spectrum is acquired. The problem arises in that NMR imaging typically performed using protons (.sup.1 H) as the nucleus of interest, while the spectroscopy is normally performed on another nucleus having a substantially different Larmor frequency, for example, phosphorous, sodium, fluorine or carbon nuclei.
Two different approaches have been practiced in the prior art for performing the two step process of first using NMR imaging to localize a specific area of interest, followed by NMR spectroscopy of the selected area. The first approach is through the use of a dual frequency coil pair. In a dual frequency coil pair, a first coil is tuned to the Larmor frequency of the nuclei to be used for imaging, while the second coil is tuned to the Larmor frequency of the nuclei to be used for spectroscopy. Prior dual frequency coil pairs have been greatly hampered by mutual losses induced between the individual coils in the coil pair. The problem is that each individual coil in the dual coil pair experiences a degradation of the coil's quality factor, Q, due to loading caused by electromagnetic coupling to the other coil in the dual coil pair, even though the other coil is tuned to a different frequency. Nevertheless, such dual frequency coil pairs have been known and used for combined imaging and spectroscopy, provided that the degradation of the results are simply tolerated. Other types of dual frequency coil pairs are known in which the individual coils in the pair are positioned such that the mutual coupling therebetween is minimized by their geometrical relationship to each other. In that case, the mutual degradation of coil Q can be reduced, but a different drawback is introduced in that each coil in the dual frequency coil pair then has a different field of view. The difference in field of view can be approximately compensated for knowing the geometric relation of the individual coils in the dual frequency coil pair, however such compensation is at best an estimate and leads to degradation of the results attainable.
Due to the aforementioned problems with prior dual frequency coil pairs, such prior dual frequency coil pairs have not been usable in practice. Instead, the alternate prevailing practice in the art is to use a first single frequency coil for performing the imaging to localize the area of interest. Then, after the imaging has localized the desired area of interest, an operator must carefully mark the position of the imaging coil, remove the imaging coil, and replace it with a second single frequency coil tuned to the frequency to be used for the spectroscopy. This procedure is obviously time consuming and tedious, and is prone to error in the placement of the second spectroscopy coil. Therefore, a need exists for a dual frequency coil pair in which each individual coil in the dual frequency coil pair has approximately the same field of view and yet is not loaded by the other coil in the coil pair.